The Outcrop for 1999, University of Wisconsin-Madison Geology Alumni Newsletter, p 34-35.

New techniques for oxygen isotope analysis have lead to the first studies of oxygen isotope geochemistry of zircons (1,2). Zircon is shown to be highly retentive of oxygen isotope ratios, preserving the best record of igneous composition even in samples that have enjoyed high grade metamorphism or hydrothermal alteration.

Figure 1

Figure 2

Zircon is a ubiquitous trace mineral (ZrSiO4) in many igneous, metamorphic, and clastic sedimentary rocks. Its ability to concentrate uranium and exclude lead forms the basis of U-Pb geochronology and its refractory nature and concentric growth patterns create robust records of crystallization age.

The ability to analyze oxygen isotope ratios in a sample that has been dated, directly links the magmatic composition to known geologic events and has opened many new and exciting avenues for study. Several recent graduate theses at the University of Wisconsin-Madison have incorporated some aspect of zirconology: Carrie Gilliam (MS 1996) and Salma Monani (MS 1999) showed that shallowly intruded, sub-volcanic Tertiary granites from the Isles of Skye and Arran, Scotland formed bymelting of anomalous rocks in the deep crust (3, 4). Liz King (MS 1997, PhD 2001) completed a comprehensive survey of Archean igneous rocks in Canada (5) that lead to a reevaluation of the genesis of one of the world's richest base metal ore deposits (6). William Peck (MS 1996, PhD 2000) is studying anorthosite and related Proterozoic granitoids in the Grenville Province and has discovered oxygen isotope provinciality that appears to identify a cryptic continental suture and an accretionary plate margin that is now buried in the deep crust (7). Ilya Bindeman, (UW postdoc) has discovered that Yellowstone's postcaldera rhyolites contain zircon xenocrysts which are zoned with respect to oxygen isotopes and age, as determined by laser abrasion, ion microprobe and SHRIMP. This provides critical evidence on zircon recycling, and the origin and potential hazard of volcanism in young calderas (10).

Another advantage of oxygen isotope studies of zircons stems from the fact that oxygen is the most abundant element in the crust. Thus oxygen is affected by different processes than trace elements or radiogenic isotopes that are commonly employed to study crustal growth and evolution. This difference is dramatically revealed by a comparison all existing data for zircons (8) from the Canadian Shield (Fig. 1). Archean age samples (2.7 to 3.0 Ga) have a very restricted range of d18O value (5.7±0.6‰) that is consistent with high temperature equilibrium with a primitive mantle reservoir and small amounts of supracrustal recycling (Fig. 2). In contrast, the Proterozoic samples from the Grenville show a much larger range and a higher average value (8.2±1.7‰). These higher values in the younger terrane result from processes that can only occur at the Earth’s surface: weathering and low temperature alteration. The high d18O signature of surfical processes must be buried in the form of sediments and altered rocks to deep in the crust in order to become incorporated in granitic magmas by melting and assimilation. Thus, the deep crust beneath the Grenville Province is dramatically different from the more primitive, early crust of the Superior Province. The quantity of high d18Osupracrustals is much smaller in the Archean. These results lead to speculation that the differences seen in North America may be worldwide trends and that sedimentation may have been less voluminous and of a different character during early Earth history. The Wisconsin group is now actively studying zircons from many other terranes, including: less than 1Ma rhyolites at Yellowstone (10),the world’s oldest known zircons from Australia (4.4 Ga, 11), and mantle-derived megacrysts from Kimberlite pipes (9). The results suggest that this trend is worldwide and that a major, non-uniformitarian change occurred at the end of the Archean (2.7 Ga).